Include group created_at attribute in file exports for offline transfer

Summary

In direct transfer, groups are imported in two steps

1. The base portable is created in BulkImports::Groups::Pipelines::GroupPipeline. This pipeline makes GraphQL queries to the source destination, which won't be possible in offline transfer.
2. Attributes for the newly created portable are updated in BulkImports::Groups::Pipelines::GroupAttributesPipeline from self relation files.

created_at is fetched in GroupPipeline and not currently present in the self relation file, so it must be included in that file in order to import the source created date in offline transfer.

Proposed implementation

Add created_at to group/import_export.yml.

Effects on other importers

• File-based group import/export would now preserve the source group's created date. We probably want this, but it should be confirmed. We may need to communicate the change even though file-based group imports are deprecated
• Direct transfer would now preserve a group's created_at, if not already fixed in #592140